1. Field of the Invention
The present invention relates to a method for setting up a communication connection between short range radio communication devices, especially Bluetooth (registered trademark) devices.
2. Description of the Related Art
In recent years, systems adopting the Bluetooth (registered trademark) specification for carrying out data transmission and reception between devices by using the frequency hopping in 2.4 GHz band have been developed for the short range radio communications.
For example, by implementing radio modules of the Bluetooth (registered trademark) specification in devices such as portable telephone terminals, PHS (Personal Handyphone System) terminals, PCs, or portable information terminals, it becomes possible to carry out transmission and reception of data between these devices. In the following, a device in which the radio module of the Bluetooth (registered trademark) specification is implemented will be referred to as a BT device.
According to the Bluetooth (registered trademark) specification, the following two procedures are necessary in the case of setting up a connection by using an ACL (Asynchronous Connection-Less) link for carrying out the packet exchange communications between the BT devices.
(1) Detecting the BT devices located in the surrounding (the inquiry procedure); and
(2) Setting up a connection with arbitrary BT device (the paging procedure).
First, by carrying out the inquiry procedure of (1), all the BT devices located in a vicinity of one BT device are detected, and the BD_ADDR (Bluetooth (registered trademark) device address), the native clock value, etc., of each detected BT device are acquired.
Next, by carrying out the paging procedure of (2) with respect to the BT device which is a communication target, a connection with this BT device is set up.
FIG. 6 shows the outline of the paging procedure. In FIG. 6, the BT device on a side that made the connection set up request with respect to the arbitrary BT device is referred to as a “master BT device 1”, and the correspondent BT device for which the connection is set up by the master BT device is referred to as a “slave BT device 2”.
When the connection set up request with respect to the slave BT device 2 is generated at the master BT device 1 (the step 11 in FIG. 6), the master BT device 1 starts the paging procedure.
The master BT device 1 that started the paging procedure repeatedly transmits the ID packet that contains a device access code of the slave BT device 2 by using different frequencies (step 12 of FIG. 6).
Note that the device access code is a code derived from the BD_ADDR, and the master BT device 1 already acquired the BD_ADDR of the slave BT device 2 by the inquiry procedure, so that it is possible to derive the device access code of the slave BT device 2.
Also, the frequencies to be used by the master BT device 1 in transmitting the ID packets are obtained as the hop frequencies that can be obtained from the BD_ADDR of the slave BT device 2.
In the case of using the 79-hop system as the frequency hopping scheme, 32 hop frequencies are selected as the hop frequencies to be used in transmitting the ID packets.
These 32 hop frequencies are grouped into two groups with 16 hop frequencies each, which will be referred to as a train A and a train B. Note that the method for selecting the hop frequencies according to the BD_ADDR is specified in the Bluetooth (registered trademark) specification.
Then, the master BT device 1 transmits the ID packets by using all of the 16 hop frequencies belonging to the train A during a period of 10 ms.
The master BT device 1 consecutively repeats this processing for a prescribed number of times (Npage), and then, repeats the processing for transmitting the ID packets by using all of the 16 hop frequencies belonging to the train B during a period of 10 ms, for the same prescribed number of times. This ID packet transmission processing is continued until a response from the slave BT device 2 is obtained.
Now, the slave BT device 2 periodically carries out the processing for receiving the ID packet containing the own device access code (step 13 of FIG. 6). In this receiving processing, the slave BT device 2 checks whether the ID packet arrives or not by scanning only one frequency selected by the own native clock value among the 32 hop frequencies derived from the own BD_ADDR.
The slave BT device 2 carries out the scanning of only one frequency selected from the own native clock value, so that the frequency that is conjectured to be scanned by the slave BT device 2 is included in the train A that contains the frequencies to be used in transmitting the ID packets first, at a time of grouping the 32 hop frequencies into the train A and the train B at the master BT device 1.
The master BT device 1 acquires the native clock value of the slave BT device at a timing of the inquiry procedure by carrying out the inquiry procedure, and maintains a difference in the clock value by comparing the acquired native clock value with the native clock value of the master BT device 1 itself.
Then, at a time of selecting the hop frequency, the native clock value of the slave BT device 2 is estimated from this difference in the clock value, and the frequency that is most likely scanned by the slave BT device 2 is conjectured.
When the slave BT device 2 receives the ID packet containing the own device access code, the slave BT device 2 returns the ID packet containing the own device access code to the master BT device 1 in response (step 14 of FIG. 6).
When the ID packet as a response from the slave BT device 2 is received, the master BT device 1 describes the BD_ADDR, the native clock value, etc., of the master BT device 1 itself as an FHS (Frequency Hop Synchronization) packet, and transmit it to the slave BT device 2 (step 15 of FIG. 6).
When the FHS packet received, the slave BT device 2 returns the ID packet as a response (step 16 of FIG. 6), and carries out the processing for synchronizing with the native clock of th master BT device 1 according to information contained in the FHS packet.
When the ID packet from the slave BT device 2 is received, the connection between the master BT device 1 and the slave BT device 2 is set up (step 17 of FIG. 6), and the data communications using this connection become possible.
Here, the master BT device 1 is going to transmit the ID packet to the slave BT device 2 repeatedly since the connection set up request with respect to the slave BT device 2 is generated until the ID packet as a response is received from the slave BT device 2.
However, the slave BT device 2 carries out the processing for receiving the ID packet transmitted from the master BT device 1 not constantly but at interval of a prescribed period.
For this reason, the ID packet transmitted from the master BT device 1 while the slave BT device 2 is not carrying out the ID packet receiving processing will not going to be received by the slave BT device 2.
For example, when the period for carrying out the ID packet receiving processing at the slave BT device 2 is 1.28 sec., as the master BT device 1 transmits 16 packers per 10 ms. the number of ID packets during that period will be:16×(1280/10)≈2000which implies at most about 2000 ID packets will be transmitted from the master BT device 1 but not received by the slave BT device 2, at a time of setting up the connection.
This causes the increase in the power consumption of the master BT device 1, and the radio resources are occupied by the transmission of the ID packets from the master BT device 1, so that there has been a problem that the communication according to the Bluetooth (registered trademark) by the third BT device have to be interrupted during this period.